1. Field of the Invention
This invention relates to a processing flow of a complementary metal-oxide semi-conductor (MOS) color filter, and more particularly to using several sequential processes of plasma, solvent, and plasma to effectively remove the complementary MOS (CMOS) color filter.
2. Description of Related Art
Currently, an image sensor generally takes an image through a charge-coupled device (CCD). The technology of CCD has been developed for many years and is mature. Compared with the CCD, another kind of image sensor, a CMOS image sensor, developed relatively late so that the resolution and quality are not yet as good as the CCD. Even so, the CMOS image sensor still has its many advantages such as lower fabrication costs due to the application of the CMOS fabrication process. The technology of the CMOS image sensor is under development, and it is predicted that the CMOS image sensor will take the place of the CCD to play the main role in the future. The CMOS image sensor is expected to be especially easy to match with the next generation of integrated circuit (IC) chips, used for image-processing or other purposes. The integration of IC devices can therefore be greatly increased. Fabrication costs are thereby brought down, dimensions are reduced, and power consumption is decreased. All these advantages enlarge the value of IC device.
In a conventional CMOS fabrication process, after the N-channel MOS (NMOS) and P-channel MOS (PMOS) are fabricated, a number fabrication processes are subsequently performed including, for example, depositing borophosphorsilicate glass (BPSG), thermal flowing, performing IC metallization, and depositing passivation. Then a bonding pad in the IC chip is exposed by photolithography and etching to ease a packaging process to complete the IC chip, which includes several hundred IC units.
The conventional CMOS used in the image sensor can only sense two colors: white or black. If it is desired for a CMOS to sense colors, the CMOS needs an additional process to form a color filter film on it with color sensitivities of red, green, and blue.
FIG. 1 is a flow diagram showing the conventional fabrication of color filter film on the CMOS device. In FIG. 1, the flow diagram starts from a step 10, in which the CMOS is fabricated including, for example, performing IC metallization and depositing Si.sub.3 N.sub.4 by plasma enhanced chemical vapor deposition (PECVD) to form a passivation layer to protect the CMOS device. In a next step 12, a planar film is formed over the CMOS device by, for example, coating. In the next step 14, three color films, including red R, green G, and blue B are formed. The planar film and the color films form a color filter film. In a next step 16, a passivation layer is formed to protect the color filter film. Then, in a next step 18, the bonding pad is exposed by photolithography and etching. In the above sequential steps, the planar film and the color films including red, green, and blue films, made of, for example, acrylic, together form the color sensing filter film.
The red, green and blue color films are of a negative photoresist type corresponding to their colors. If these color films are exposed, a polymer is formed. The polymer can not be completely removed by plasma or solvent, which are typically used in the CMOS fabrication process. On the other hand, if the color films are not exposed and removal is the color films of the color filter film can be removed by a developer. However, after a high temperature curing process, the planar film cannot be removed by the solvent usually used in a CMOS fabrication process.
If a fabrication error is found, the acrylic color filter film, which is also a kind of photoresist, must be removed. This is called reworking. In order to remove the photoresist, one may make use of the conventional patterning photoresist method to remove the acrylic color filter film but some problems persist.
FIG. 2 is a flow diagram schematically illustrating the conventional processes for patterning the photoresist layer used in CMOS fabrication. In FIG. 2, the flow diagram starts from a step 20, in which the photoresist pattern is patterned. If the photoresist has not been pre-baked at a step 22, the photoresist can be removed by a solvent such as acetone, as show in a step 24. If photoresist has been pre-baked at a step 26, then a plasma process is necessary to remove most of the photoresist as stated in a step 28. Then, a solvent removes the residual photoresist as shown in a step 30.
As described above, the conventional patterning photoresist method can remove the photoresist but it can not effectively be applied to removing the color filter film because the color filter film is made of acrylic. For example, the plasma process, no matter how long it is applied, can not completely remove the acrylic photoresist. So, a flaw occurs in the subsequent fabrication process and causes the rejection or failure of the IC chip.